The present invention relates to an improved hip prosthesis.
Prosthetic replacement of diseased and/or damaged hips, though not yet totally perfected, is a widely accepted surgical approach which has received a significant degree of success. In general, many years ago, surgeons began to surgically implant one piece hip prostheses having a stem and head where the head articulates within the natural acetabulum or an acetabular cup, while the stem of the prosthesis extends downwardly into the medullary canal. Initially, such was achieved by an elongated stem which was simply inserted directly into the marrow cavity. The distal tip of the prosthesis resided in soft marrow tissue and could move laterally from side to side. As the patient walked, the prosthesis was subject to toggling, whereby the prosthesis loosened within the femur, leading to resorption or other destruction of the supporting bone. Further attempts have since been made to create a prosthetic device that could be successfully implanted in a patient's hip without the subsequent loosening effect. In general, loosening of the prosthesis normally leads to a characteristic course of deterioration of the bone, resulting in serious loss of hip function and/or pain.
Under impingement fixation as mentioned above, the distal portion of the stem could move laterally to and fro when the patient walked. This movement approximated rotation about the connection between the stem and the head in the calcar region. After prosthesis failure, further surgery or patient inactivity resulted. In order to overcome the distal tip movement, loosening of the prosthesis and the consequential "windshield wiper effect", dental bone cement, which is in essence a chemical composition which polymerizes in situ within the medullary canal, was placed about the stem of the prosthesis within a slightly reamed medullary cavity to block the lateral movement of the stem within the medullary canal. Initially, only small amounts of bone cement were employed with realization of great improvement. Thereafter, surgeons further reamed the medullary canal and inserted greater amounts of bone cement according to the thesis that a greater bond, and thus more permanent fixation would result. While a more rigid initial fixation resulted, after prolonged periods of time, calcar bone resorption and fracture of the bone cement often occurred, leading to a recurrence of pathological processes, and perhaps replacement of the prosthesis.
Further, during polymerization of bone cement, generally a polymethylmethacrylate polymer, a significant exotherm is generated and less than total polymerization is achieved. Residual toxic monomeric substances thus remain in the medullary canal. Both the exotherm and the monomeric materials can produce adverse effects.
Still further, in order to approximate the normal anatomical shape to the femur, modern hip prostheses have conventionally included curved and tapered stems in order to facilitate insertion, while also avoiding the necessity of inordinate reaming of the femur. Such tapered shapes, in essence act as a wedge, such that subsequent to implantation, normal activity creates forces against the prosthesis which are transmitted via the tapered stem to the femur in undesirable directions in certain localized conditions. Particularly, with continued activity and some loosening of the prosthesis, loss of calcar bone results due to resorption. A loss of axial compressive force on the calcar is also believed to lead to resorption of the calcar bone, further compounding the loosening problem. The distal tip of the prosthesis stem engages a bolus of bone cement located therebeneath, and a reduction of applied forces at the calcar results in an increase in axial compressive forces from the distal tip to the cement. Upon receipt of adequate force, the cement distal to the prosthesis tip fractures, permitting subsidence of the prosthesis and/or forcing of the polymerized bone cement further into the canal, again resulting in excessive loosening. Furthermore, the "wedging" resulting from tapered prostheses stems accounts for decreased axial compressive strains and increased tensile hoop stresses in the calcar. Also, at the level of the prosthesis tip, both axial compressive load and bending moments act on the femur, being transferred entirely to the bone at this level, and resulting in regions of increased strain.
The above noted altered strain patterns on the calcar is a major factor leading to remodeling of the bone and resorption of the calcar since the calcar bone is no longer required to support the entire joint load as with a normal, healthy femur. Particularly, a change in stress orientation from predominantly axial compression to predominantly circumferential tension creates stress across the grain of the bone as opposed to along the grain as provided by nature.
A number of factors have thus been proposed as possible contributory causation to prosthesis loosening, namely stress shielding of the calcar femorale; surgical impairment of blood supply in or around the implanted prosthesis; necrosis of bone tissue due to the exotherm produced during polymerization of bone cement; necrosis of bone tissue due to toxic substances released from the bone cement; relative movement at the bone cement interface during activity; and mechanical failure of the cement, particularly in the calcar region. Further and quite importantly, particulate matter produced from wear, bone erosion cement fracture and the like can develop after implantation, become entrapped within the reamed medullary canal, and provoke persistent local inflamation.
The hip prosthesis according to the present invention overcomes at least certain of the aforementioned problems by way of a unique prosthesis design which better approximates physiological axial compressive loading of the calcar while avoiding wedging and axial load transfer from the distal tip of the prosthesis stem to the femur. While the hip prosthesis of the present invention is implanted with the use of a luting agent such as bone cement, the instant prosthesis is manufactured to preclude load transfer to the luting agent by shear stresses acting across the prosthesis-luting agent interface or by direct impingement of the tip on the subjacent luting agent. Particularly, the design of the prosthesis according to the present invention permits continual contact between the prosthesis and the calcar bone to generate the approximate axial compressive stresses therein. The stem of the prosthesis is also provided with a generally uniform cross section such that only minimal wedging is experienced.
There is no known prior art that is believed to anticipate or suggest the hip prosthesis according to the present invention. Exemplary of the known prior art are U.S. Pat. Nos. 2,785,673; 3,879,767; 3,938,198; 4,012,796; 4,051,559; 4,068,324; 4,227,265; and 4,280,233.